This invention relates to vehicular skid control systems and more particularly to an improved fail safe device for such a system.
Automatic skid control systems include what is generally known as a modulator that is interposed in the circuit between the master cylinder and the wheel cylinders. This modulator is effective to reduce the pressure transmitted from the master cylinder to the wheel cylinders in the event of an incipient skid condition. The modulator includes one or more members that are movable between a normal braking position and the skid control position. Some form of power device is incorporated for moving the modulator member between these positions. In certain systems, this power device is effective to hold the movable member in its normal braking position. In the event of failure of the power device, the movable member may move to its skid control position and interfere with normal brake operation. It is, of course, desirable to provide a fail safe device that will insure normal brake operation in the event of such a failure. Devices have been proposed for this purpose. In at least certain of these prior art devices it was necessary for the operator to apply master cylinder pressure to operate the fail safe device. Such an arrangement can result in the loss of at least some brake actuating fluid, causing excess brake pedal travel. Furthermore, the systems are not automatically preconditioned for fail safe operation in the event of failure.
It is, therefore, a principal object of this invention to provide an improved fail safe device for a vehicular skid control system.
It is a further object of the invention to provide a fail safe device for a skid control system that permits normal brake operation under the failure mode.
It is another object of the invention to provide a skid control system fail safe device that permits the transmission of actuating fluid from the master cylinder to the wheel brakes independently of the expansion chamber of the skid control system.
It is a still further object of this invention to provide a fail safe device for a skid control system that is automatically operative to provide fail safe operation in the event of failure of the skid control system without necessitating operation of the vehicle brakes to establish this condition.
In addition to providing a separate actuating circuit between the master cylinder and the wheel cylinders under the failure mode, it is desirable to isolate the master cylinder from the modulator in the event of failure. Doing so insures against the loss of actuator fluid into the modulator under failure conditions. The previously proposed systems that accomplish this result employ complicated multiple valve arrangements.
It is, therefore, a further object of this invention to provide an improved bypass valve for a skid control system.
As has been noted, some form of power device is normally provided in a skid control system for operating the modulator between its skid control and normal braking positions. The advantages of using am amcillary fluid pressure source, such as the power steering pump, for this purpose have recently been emphasized. In one type of system embodying the use of such ancillary fluids, the ancillary fluid holds the modulator in its normal braking position. It is, as has been noted, important to insure normal brake operation in the event of failure of the ancillary fluid source to provide sufficient pressure to hold the modulator in its normal braking position. This fail safe device should be operative immediately upon such failure.
It is, therefore, another object of this invention to provide a fail safe device for a fluid pressure operated skid control system that is operative immediately upon the failure of the fluid pressure operator.